In recent years, along with the widespread use of the fuel injection device called an injector, control of fuel injection timing and injected fuel amount or an air-fuel ratio has become easy. As a result, it has become possible to increase output, reduce fuel consumption, and clean exhaust gasses. As for the fuel injection timing in particular, in strict terms the state of the intake valve or generally the camshaft phase is detected and fuel is injected according to the detected value. However, the so-called cam sensor for detecting the camshaft phase state is expensive and in most cases the cam sensor cannot be employed particularly in a motorcycle because of the problem of enlarged cylinder head. For that reason, a proposal of an engine controller is made, for example, in the patent publication JP-A-H10-227252, in which the crankshaft phase state and the intake air pressure are detected and from the detected values the cylinder stroke state is detected. Therefore, using the above-mentioned prior art, the stroke state can be detected without detecting the camshaft phase, so that fuel injection timing can be controlled according to the stroke state.
Incidentally, in order to control the amount of fuel injected from the above-described fuel injection device, for example it is possible to set a target air-fuel ratio according to the engine speed or the throttle opening, detect the actual intake air flow rate, and calculate a target fuel injection amount by multiplying the inverse of the target air-fuel ratio.
For detecting the intake air flow rate, a hot wire air flow sensor and a Karman vortex sensor are generally used to measure the mass flow rate and the volumetric flow rate, respectively. However, a volumetric member (surge tank) for restricting pressure pulsation is required to eliminate error factors due to reverse air flow, or it is required to install the sensor in a position the reverse air flow does not reach. However, most motorcycle engines are of the so-called independent intake type or the single cylinder type. Therefore, the above requirements cannot be met satisfactorily with most of the motorcycle engines, and the intake air flow rate cannot be detected accurately with the flow rate sensors mentioned above.
Another problem is that, since detection of the intake air flow rate is made at the end of an intake stroke or in the early period of a compression stroke when fuel has already been injected, the air-fuel ratio control using the intake air flow rate is effective only in the next cycle. This means that, before the next cycle, the air-fuel ratio is controlled according to the air-fuel ratio of the previous cycle in spite of the driver intending to accelerate and opening the throttle. Therefore, the driver will have a feeling of inconsistency because of insufficient acceleration due to insufficient torque or output. To solve such problems, the driver's intention for acceleration should be detected by detecting the throttle state using a throttle valve sensor or a throttle position sensor. However, such sensors cannot be employed especially in a motorcycle because of their large size and high price, and the problems remain unsolved at the moment.
Therefore, the following arrangement can be devised: the crankshaft phase and the intake air pressure in the intake pipe of a four-stroke engine are detected; an accelerating state is determined to be present when the differential value in the intake air pressure at the same crankshaft phase in the same stroke between the current cycle and the previous cycle is not smaller than a specified value; when an accelerating state is determined to be present, fuel is injected immediately from a fuel injection device, for example, so as to respond to the intention of the driver to accelerate. Here, smooth changes in the intake air pressure according to the stroke are required on one hand, and real changes in the intake air pressure are required when detecting the intake air flow rate on the other. In other words, intake air pressure changes that are smooth but real according to the stroke are required for detecting the accelerating state and the intake air flow rate of the engine, or the load. However, the presence of vibration in the intake air pressure detected with the pressure sensor has become known in addition to simple electric noises. The vibration hinders the detection of the intake air pressure changes according to the stroke.
The present invention has been developed to solve the above problems, with the object of providing an engine controller, which detects the engine load from the intake air pressure, controls the engine operating state according to the engine load, and can securely detect changes in the intake air pressure corresponding to the strokes during the control.